Without any hesitation i can say that what i see looks like ISO 6400 on my 5D mk II

although, for me, i still didn't like ISO 6400 on the 5DMkII, if +2EV to ISO25600 are very close on the 5DMkIII...

then it means my 'favorite' baseline ISO 800 (on my 5DMkII; which is quite flexible and clean), can be shifted upwards to ISO 3200 (on a 5DMkIII), which I never was too keen on, even on my 5DMkII, even though I could easily use them; i preferred not to. (even restricting myself to ISO 1000 - 1250 was more likely; and 1600-2500 'tolerable')

i suppose given most folks expect image reductions for presentation, i could manage with what i see for 5DMkIII ISO25600 (being close to 5DMkII ISO6400)

thus, where i opted to hone in on ISO 800 moonlit shots @ 30" before (for 'moony' bright clear blue sky starry nights), it was easy for landscapes only, but for landscape portraits (especially self-portraits 'solo'), it was hard to 'hold' for 30"!!!

if i push the 'acceptability' to ISO 6400 (5D2) or ISO 25600 (5D3), it means i can now reduce 'portrait landscape' times down from a long 30" to at least:

for f2.8:
800/1600/3200/6400/12800/25600
30/15/8/4/2/1... 4 seconds on a 5D2... and 1 second on a 5D3

for f/9 - f/11 (for improved DOF of 'near-far' landscape portraits):

f2.8/4/f5.6/8/f11
that's pushes times back longer:
1/2/4/8... or 4-8 seconds...

this is why i feel, although this is definitely an expanded improvement for my shooting, i definitely want it to go further, to 'good ISO 102400+'

25600/51200/102400/204800
bringing times back down for f/9-f/11 from:
4-8 seconds... 8/4/2/1... back down to a more manageable 1"...

all NOT relying on flash-fill... and ensuring ZERO 'star-streaks'... which is what i really want for hgih DOF moonlight starry night landscape portraits (super calm waters, bright blue cloudless skies...) and if i am lucky, colorful auroras along with an expansive 'still' milky way... (i'm not there yet; especially if i want it 'straight-out-of-the-camera).

in this sense, it is the 5DMkIII's added HDR/M.E. (multiple exposure) feature that may compensate to combine 'tolerably' long exposures, and 'compromise' with 'flash-fill' shots captured in shorter 'portrait times'... not much choice on that, at least for now.